sdcexp-1(position and speed control)

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NGEE ANN POLYTECHNIC

Diploma in Aerospace Technology

System Dynamics & Control

EXPERIMENT (SDCEXP-1): POSITION & SPEED CONTROL OF DC MOTOR

SYSTEMS

OBJECTIVE:To enable the student to understand the following concepts:

(a) Open loop and closed-loop control of position and speed

(b) Velocity feedback in position control

(c) Effects of increasing system gain

(d) Effects of load changes on a speed control system

EQUIPMENT: Servo Demonstrator SD 155

MS-I 50 Feedback Control System

Hand-held tachometer (optional)

220 KOhm resistor block

INTRODUCTION:

In Position Control, the objective is to maintain (control) the physical position (linear or angular)of the mechanism. For example, an electric motor could be used to change the position of a

rudder of an airplane, to operate a lift, to alter the launch angle of a missile or to position a

robotic arm. In Speed Control systems, the controlled variable is speed (again, either linear or

angular). They are used in continuous processes such as sheet material rewinder systems,

synchronisation of shaft rotation in printing and hot rolling mills, and in applications such asmissile guidance systems, automatic piloting systems, lifts and overhead hoist control.

Control can be achieved either in open or closed loop. In an Open Loop control system, no

mechanism for implementing feedback exists. Hence if the actual position deviates from thedesired position, no automatic corrective action will take place. In a Closed Loop control system,

the system is error actuated. This means that the systems actuator output depends on the

difference between the actual controlled value (feedback) to the desired value (setpoint). Withclosed loop control, the system can automatically compensate for changes in the load. At higher

loads, more energy will be supplied to maintain the controlled variable at the setpoint.

Due to system inertia, a position control system with only position feedback, may exhibit an

undesirable oscillatory output. This is because the output position tends to overshoot its desiredposition. Oscillations can be dampened out if the system is able to reverse its torque before the

position error reaches zero. This can be achieved by adding to the position error signal another

signal, which would counter-act the error signal when required, but diminish when the error hasbeen reduced to zero. The velocity signal is normally used as the counteracting signal. Velocity

feedback, when applied to a closed-loop position control system will have a dampening effect on

oscillations in the position variable.


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PART 1- POSITION CONTROL PROCEDURE / OBSERVATION:

PROCEDURE/OBSERVATION :

A - OPEN LOOP POSITION CONTROL

1. Connect the wires from the servo panel to the motor (paper roller) according to theirrespective color codes.

2. Disconnect the yellow lead (Y) from the position feedback potentiometer.

3. Plug in Re, 10 K only. Do not plug in Rvel.

4. Lift up the pen-holder, switch on the power supply to the motor and the servo panel. Do notactivate the paper roller.

5. Try to adjust the INPUT potentiometer until the OUTPUT (motor shaft/aileron position ) is in

the middle position. Is this possible ? Why?

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B - CLOSED LOOP POSITION CONTROL

1. Without switching off the power supply, connect the Yellow lead (Y) from the Position

Feedback potentiometer.

[Notice that the motor shaft/aileron is now oscillating. Continuous oscillations can wear

out the feedback potentiometer. Hence, do not leave the system running for more than 10

seconds continuously! Switch off the power after each step is completed]

2. Set the input potentiometer to position 0. Vary the INPUT signal (desired value) by changing

the position potentiometer (from 0 to 4 and 0 to -4). Notice that the motor shaft/aileron

follows the changes in the input this time.

3. Lower the pen-holder and switch on the paper roller. Apply a STEP INPUT to the system by

quickly turning the potentiometer from 0 to 4. Plot the response curve on the paper roll.

4. Switch off the power supply, stop the paper roller and lift up the pen-holder.

5. Tear out the response curve, stick the response curve at the space provided.

Please cut and paste the response curve here;

C - CLOSED LOOP POSITION CONTROL WITH VELOCITY FEEDBACK

In this part of the experiment, anadditional feedbacksignal calledVelocity Feedbackis added.It has the effect of increasing the damping in the system.

1. Plug in 5.6 K resistor as the Rvel to provide for velocity feedback.

2. Turn the INPUT potentiometer to 0 position and switch on the power supply.

3. Lower the pen-holder and switch on the paper roller. Input a STEP INPUT to the system by

quickly turning the INPUT potentiometer to 4 position .Plot the response curve on the

paper roll. Stop the paper roller and label the resistor values on the response recorded.Please cut and paste the response curve here;


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4. Increase the velocity feedback by changing the value of the Rvel resistor to l KOhms. Repeatstep (3) of this section. Note the change in the speed of response. Is the speed of response

faster or slower?

Please paste the response curve here;

5. Decrease the velocity feedback by changing the value of the Rvel resistor to lOO KOhms andrepeat step (3). Note the change in speed of response as compared to the plots taken in the

previous two steps. Is the response faster or slower?

Please paste the response curve here;

D - CLOSED LOOP POSITION CONTROL WITH INCREASED GAIN

In this experiment set-up, the error signal is amplified before reaching the motor. If the gain of

the amplifier is high, the system will become more sensitive, resulting in a faster system

response.

1. Plug in 5.6K resistor into the Rvel (for velocity feedback).

2. Set the INPUT potentiometer to 0 position.

3. Increase thegainby changing the value of theRe resistor to 1K (Re= Gain). Switch on the

paper roller. Give the system a step change at the input andtake a plot of the system response .Switch off the paper roller and label the resistor values on the plot taken.


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4. Decrease the gain by changing the value of the Re resistor to lOOK andtake another plotof a step response. Note the difference in speed of response between the two gain settings. Is

this response faster or slower than that obtained in the previous step (3)?

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Please cut and paste the response curve here;

5. Switch off all the power supply to the Servo Demonstrator and disconnect all the resistors.


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PART 2 - SPEED CONTROL

A - OPEN LOOP SPEED CONTROL

In this section, we shall study the effect of load changes on a speed control system operated

under open-loop control.

1. Set up the Speed Control apparatus for the MS-ISO system as shown in figure 2.

2. Disconnect the tacho feedback signal from the Op Amp Unit and swing the magnetic brakesclear from the brake disc. Ensure that the feedback selector is turned to External Feedback.

3. Set the Input Attenuator to 0. Turn on the power supply and adjust the Attenuator until themotor speed is about 1000 rpm. Use the hand-held tachometer to register the speed. [ not

allow the motor current to exceed 2 Amps/

4. Stop the motor by turning off the power supply. Swing in the magnetic brakes until the edge of

the magnets coincides at the 4th mark.

5. Restart the motor and record again the motor speed in Table 1.

Initial Speed (rpm)

( o oa

Final Speed (rpm)

( t oa

% Speed Reduction

Open Loop

Closed Loop

B - CLOSED-LOOP SPEED CONTROL

The procedure carried out in the previous section is repeated with the system under closed- loop

control.

1. Connect the tachogenerator feedback signal back to the Op Amp Unit and swing the magneticbrakes clear from the brake disc.


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2. Repeat the steps (3) - (5) in the previous section A , recording your results again in table l.

DISCUSSION:

Part 1 - Position Control

1. Attach all the plots taken from the Servo Demonstrator into your report. Ensure each plot is

neatly labeled with their respective resistor values (Reand Rvel). With reference to these plots,answer the following questions:

a. Why is the output of the Closed-loop position control system (without velocity feedback) so

oscillatory?

b. Comment on the improvement in position control after velocity feedback is added into thecontrol system.

c. How does the gain setting affect the speed of response? What would you expect if the gain is

set too high?

d. From all the plots taken, which combination of Re and Rvel would you consider to give the

best output for position control? Explain your answer.

Part 2 - Speed Control

Using the results recorded in table 1, comment on the effect of load change under the open andclosed-loop control. Suggest one method to further reduce the percentage drop in speed under

closed-loop control.

CONCLUSION

Briefly conclude on the findings of this experiment.

sdcexp-1(position and speed control)

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